1. Field of the Invention
This invention relates to a semiconductor laser driving apparatus. More particularly, the present invention relates to improved technique of a driving apparatus for controlling the irradiating energy or the intensity of a laser beam (optical output) from a semiconductor laser to obtain a continuous gradation.
2. Description of the Prior Art
There has been known conventionally a laser printer or the like which makes intensity modulation of a laser beam generated by a semiconductor laser (laser diode; LD) by an optical modulator disposed outside and effects exposure scanning on a photo-sensitive material to obtain tonal images. The following method is known which obtains images having continuous gradation (not the images by the Dither method but those images in which one pixel has density information) on a photosensitive material by directly controlling a current to be supplied to the semiconductor laser so as to control the optical output of the semiconductor laser without using the external modulator described above.
The semiconductor laser has predetermined characteristics between a current supplied thereto and its optical output. Therefore, if the current is controlled by a degree corresponding to a required gradation, the optical output can be controlled directly without using the external modulator, and 256 (2.sup.8) gradations, for example, can be obtained by dividing the current by 256 steps.
Continuous gradation can be obtained, too, by keeping constant the current to be supplied to the semiconductor laser as well as its optical output and variably controlling the pulse width of one pixel clock. When, for example, one pixel clock is 300 ns (maximum exposure time), time resolution of 300 steps can be obtained by controlling the pulse width from 1 ns to 300 ns by every 1 ns, and as high as 300 continuous gradations can be obtained by the exposure time control of the 300 steps (refer to Japanese Patent Laid-Open Nos. 152372/1981, 58068/1986), though it depends on the property of the photosensitive member.
In case that the pulse width of one pixel clock is controlled as described above, the pulse width can be changed, too, by analog processing besides the digital processing described above.
Furthermore, optical quantity levels of 2.sup.N levels can be obtained by preparing N current sources having mutually different current values as current sources for the semiconductor laser and combining them digitally (Japanese Patent Laid-Open No. 184773/1988).
A multi-stage gradation can also be obtained by modulating the optical output of the semiconductor laser together with the exposure time as shown in (Japanese Patent Laid-Open No. 124921/1986).
In accordance with the structure for obtaining the continuous gradation by dividing and controlling the current in accordance with the required number of gradations as described above, however, the current difference in the optical output range is 14 mA if the output characteristics of the semiconductor laser used are such as a dot line shown in FIG. 35 and the range of use of the optical output is from 0 to 3 mW, for example. If the gradations of 256 steps must be obtained here, the current must be controlled with accuracy of 14 mA/256=55 .mu.A. Therefore, if a structure shown in FIG. 36 which converts digital input image data by D/A converters and supplies a current to a semiconductor laser (LD) through an amplifier, high speed and high precision D/A converters must be employed as the D/A converters. Thus, there remain the problems that the cost of apparatus becomes high and necessary accuracy cannot be secured so easily.
When the pulse width of one pixel clock is controlled variably by a digital processing, division (unit increase time) of the pulse width must be set finely in order to obtain sufficient gradations only from the pulse width. When gradations of 1,024 steps, for example, must be obtained, division of the pulse width becomes 300 ns/1,024=0.3 ns if one pixel clock is 300 ns, and time resolutions of a GHz order is required. It is difficult to attain such time resolution by ordinary circuit technique, and the method of variably changing the pulse width described above is effective when the required frequency of the pixel clock is low (in the KHz order) but its practically drops when the required frequency becomes high.
When the variable control of the pulse width is made by the analog processing as described above, a triangular wave in synchronism with the pixel clock may be generated and is compared with the analog value of the input data to convert it to the pulse width. Thought the high frequency pulse is not necessary in this case, a triangular wave which has an accurate slope and is therefore difficult to generate must be generated and this method is inferior in the aspect of accuracy to the digital processing.
In the method of obtaining the continuous gradation by use of a plurality of current sources, 10 current sources are necessary to obtain the gradation of 1,024 (2.sup.10) steps, for example, so that the circuit becomes complicated and the increase in the cost of production is unavoidable. In the method which uses a plurality of current sources such as the one disclosed in Japanese Patent Laid-Open No. 184773/1988, for example, the characteristics of the optical output of the semiconductor laser and those of the current are assumed to be those shown in FIG. 37. However, whereas the optical output hardly changes within a range (natural emission region) below a boundary current (threshold current) at which the semiconductor laser starts oscillation as shown in the afore-mentioned FIG. 35, the optical output increases abruptly within a range (laser oscillation region) beyond this boundary current. For this reason, it cannot be said that the optical quantity levels of 2.sup.N can always obtained when the number of current sources is N.
If the current is divided (into I.sub.0, I.sub.1, . . . ) so that the optical output has equidistant gaps .DELTA.Po as shown in FIG. 38, for example, I.sub.0, I.sub.1, I.sub.2 in the non-linear region are not equal to one another while I.sub.3 .about.I.sub.7 in the linear region are equal to one another. Accordingly, it is not possible to make the control (the control based on the premise that the relation between the optical output and the current in linear) by assuming the least significant bit number (LSB) used for ordinary D/A converters or the like is A, the next bit number is 2A and so forth with subsequent bit numbers being 4A, . . . , 2.sup.N-1 and by combining these units to obtain 2.sup.N optical outputs.
In the example shown in FIG. 38, LSB=I.sub.0 but a current corresponding to the input data 2 is I.sub.0 +I.sub.1 .noteq.2I.sub.0, a current corresponding to the input data 3 is I.sub.0 +I.sub.1 +I.sub.2 .noteq.3I.sub.0 and furthermore, a current corresponding to the input data 4 is I.sub.0 +I.sub.1 +I.sub.2 +I.sub.3 .noteq.4I.sub.0. Accordingly, 2.sup.N optical outputs cannot be obtained from (N-1) units of A, 2A, 4A, . . . , 2.sup.N-1 A described above.
In an embodiment of the Japanese Patent Laid-Open No. 124921/1986, for example, the optical output of the semiconductor laser and the exposure time are modulated together, and input digital data are divided into lower order bits and upper order bits, so that the pulse width is controlled by the lower order bits and the optical output (supply current) is controlled by the upper order bits. In this case, waveforms of current flowing through the semiconductor laser are shown in FIGS. 39 and 40, however, there is a problem whether the simplicity of the density actually obtained when the optical output is switched can be guaranteed or not.
Specifically, in case of FIGS. 39 and 40, it is difficult to obtain stably a relation of "P.sub.1 .times.density obtained during the maximum time to (1 pixel time).ltoreq.P.sub.2 .times.density obtained during the minimum time .DELTA.t" in consideration of the fluctuation or change with time in property of the photosensitive material, and the change with time of process etc. Accordingly, the gradation property is deteriorated and the density can not be reproduced with a high fidelity, if the simplicity of density can not be obtained.